Diagnosing Thin Shots
Not normally a factor in thin shots.
Your shoulders could be pointing well right or well left at address. This puts the bottom of the swing in the wrong place.
Look for a large deviation from the norm. A driver's ball position should be around the front heel, moving progressively farther back until it reaches the middle of the stance with short irons (photo).
The club may be deviating from its gentle arc on the backswing, tracking a path that's either too much inside or too much outside. Posture should remain constant without raising up.
No effort should be made to lift the ball into the air by pulling your arms up through impact. Check to make sure the circle of your swing is in the right place by making practice swings to see if you can hit the ground slightly after the ball. (Irons are design to strike the ball with a descending blow - see Hit Down, Dammit! for more on this concept.)